Update hosts.yaml (ansible#24513)

Example to clarify the rule :
``` Anything defined under a hosts is assumed to be a var```

examples/hosts.yaml

@@ -17,6 +17,7 @@
 ##  hosts:
 ##      green.example.com:
 ##          ansible_ssh_host: 191.168.100.32
+##          anyvariable: value
 ##      blue.example.com:
 ##      192.168.100.1:
 ##      192.168.100.10: